1. Field of the Invention
The present invention relates to vinyl alcohol polymers and compositions thereof. More specifically, the present invention relates to vinyl alcohol polymers having good thermal stability, water resistance, gas-barrier properties, water vapor-barrier property, aqueous solution stability when kept at low temperatures, and biodegradability.
2. Description of the Prior Art
Polyvinyl alcohol (hereinafter sometimes referred to as xe2x80x9cPVAxe2x80x9d), being one of a few crystalline, water soluble polymers, has excellent film formability, transparency, strength properties and interfacial activity. The polymer has therefore been widely used as a paper modifying agent such as coating agent for paper or additive for paper manufacturing process, an adhesive for paper, wood, inorganic materials and the like, a sizing agent for filling, a stabilizer for emulsion polymerization and suspension polymerization and as various binders. Furthermore, polyvinyl alcohol is important as a raw material for films, sheets and the like made of polyvinyl alcohol.
xe2x80x9cCompletely saponified PVAxe2x80x9d having a degree of saponification of about 98 mole % and xe2x80x9cpartially saponified PVA having one of about 88 mole % are known as conventional PVAs. In addition, development of high-performance PVAs by introduction of some functional groups to improve specific properties has been performed and, as a result, various modified polyvinyl alcohols are available.
Conventional PVAs, having not so high thermal stability, have been used in the form of aqueous solutions. That is, xe2x80x9ccompletely saponified PVAxe2x80x9d is not melt moldable because its melting point and thermal decomposition temperature are very close to each other. On the other hand, xe2x80x9cpartially saponified PVAxe2x80x9d, which has a melting point lower than that of xe2x80x9ccompletely saponified PVAxe2x80x9d, has low thermal stability and hence has the problem of generating acetic acid odor on melt molding.
There has been proposed blending a plasticizer or other polymers with PVA to decrease the melt viscosity of the PVA, thereby rendering the resulting PVA to become melt moldable. However, with the molded articles obtained after addition of a plasticizer, the content of the plasticizer decreases with time, when they are used over a long period of time. Then, under low-temperature and low-humidity conditions such as in the winter season, the molded articles become insufficient flexibility and tend to suffer from generation of splits or cracks. Also, blending of other polymers with PVA tends, due to poor compatibility between the blended polymers, to markedly decrease the mechanical properties or transparency of the molded articles.
Modification of PVA to decrease its melting point has also been proposed.
Thus, Japanese Patent Publication Nos. 10885/1992 and 49683/1993 proposed, respectively, a PVA having units from a xcfx89-hydroxyalkyl vinyl ether and those from an alkyl vinyl ether and a PVA having units from a polyoxyethylene monoallyl ether. These modified PVAs however suffered from poor thermal stability. Japanese Patent Application Laid-open No. 229135/1987 proposed a PVA having allyl alcohol units, which was improved in thermal stability to some extent but insufficiently for practical purposes and, moreover, had the problem of safety of the allyl alcohol remaining in the PVA. Japanese Patent Application Laid-open No. 289581/1988 proposed a PVA having units from an xcex1-olefin, which tended to suffer from a marked increase in the melt viscosity due to association of the hydrophobic groups and further had the problem of being insoluble in water. Japanese Patent Application Laid-open No. 228625/1995 proposed a PVA having on the side chains thereof a hydroxyalkyl group with specific number of carbon atoms, which was improved in thermal stability to some extent but insufficiently for practical purposes.
PVAs also have the advantages of having excellent gas-barrier properties and transparency and causing little waste treatment problems, so that films utilizing xe2x80x9ccompletely saponified PVAxe2x80x9d are sometimes used as gas-barrier layers. However, it is known that although PVA films have high gas-barrier properties at low moisture absorption, i.e. under dry atmosphere, they strongly absorb moisture to become of low gas-barrier properties under a relative humidity of about 70% or more. In order to decrease the moisture absorbing tendency of PVA, there has been used ethylene-vinyl alcohol copolymer (hereinafter referred to as xe2x80x9cEVOHxe2x80x9d) having copolymerized at least 20 mole % of ethylene. EVOH however is insoluble in water, so that for use in the form of solution it should be dissolved in an organic solvent, which markedly worsen the work environment. There have also been proposed, in order to decrease the moisture absorbing tendency of PVA, use of modified PVAs having crosslinking property, use of PVA reacted with a coupling agent, and use of PVA reacted with another polymer to introduce crosslinking structure. However, these modified PVAs have still insufficient gas-barrier properties under high-moisture conditions perhaps due to formation of pores on crosskinking.
Accordingly, an object of the present invention is to provide a vinyl alcohol polymer which can solve all the inherent problems of conventional PVAs and thus has excellent thermal stability, water resistance, gas-barrier properties and, when it is used in the form of aqueous solution, has good stability when the aqueous solution is allowed to stand for a long time at low temperatures.
Another object of the present invention is to provide a vinyl alcohol polymer composition having excellent thermal stability, water resistance, gas- and water-vapor-barrier properties, stability when its aqueous solution is kept at low temperatures, and biodegradability.
As a result of an intensive study to develop a vinyl alcohol polymer having the above favorable properties, the present inventors found a vinyl alcohol polymer having an ethylene unit content of 2 to 19 mole %, a polymerization degree of 200 to 2,000, a saponification degree of 80 to 99.99 mole % and a total content of carboxyl group and lactone rings of 0.02 to 0.4 mole %, and completed the invention.
It is necessary that the vinyl alcohol polymer of the present invention have ethylene units. The content of ethylene units should be 2 to 19 mole %, and is preferably 2.5 to 17 mole %, more preferably 3 to 15 mole %, most preferably 3.5 to 13 mole %. If the ethylene unit content is less than 2 mole %, the above thermal stability, water resistance, gas-barrier properties, water vapor-barrier property, stability of aqueous solution when kept at low temperatures and biodegradability will be improved only to small extents. On the other hand, if the ethylene unit content exceeds 19 mole %, the water solubility, which is the key feature of PVA, will decrease.
The content of ethylene units in the vinyl alcohol polymer of the present invention is determined by proton NMR on the polyvinyl ester having ethylene units, which is the precursor of the vinyl alcohol polymer. The obtained polyvinyl ester is sufficiently purified by at least 3 processes of reprocipitation from n-hexane/acetone and then vacuum dried at 80xc2x0 C. for 3 days, to yield a polyvinyl ester for analysis. The polymer is dissolved in DMSO-D6 and the solution is subjected to 500 MHz proton NMR (with GX-500, JEOL) at 80xc2x0 C. The ethylene unit content is obtained by calculation from a peak (4.7 to 5.2 ppm) originating from the main chain methyne of the vinyl ester and peaks (0.8 to 1.6 ppm) from the main chain methylene of ethylene, vinyl ester and the third component used.
The vinyl alcohol polymer of the present invention has a viscosity average degree of polymerization (hereinafter referred to simply as xe2x80x9cpolymerization degreexe2x80x9d) of 200 to 2,000, preferably 220 to 1,800, more preferably 240 to 1,600, most preferably 250 to 1,500. If the polymerization degree is less than 200, it will become difficult to process the polymer into films and other molded articles and to obtain satisfactory products. Furthermore, the obtained products will have low mechanical strength, thus shrinking from the feature of PVA. On the other hand, if the polymerization degree exceeds 2,000, the melt viscosity on melt molding or, on processing of an aqueous solution of the polymer, the viscosity of the aqueous solution will become high, so that the operability and processability become worse and satisfactory products cannot be obtained.
The polymerization degree (P) of vinyl alcohol polymers is determined in accordance with JIS-K6726, as follows. A vinyl alcohol polymer sample is re-saponified and, after purification, tested for the intrinsic viscosity [xcex7] (dl/g) in water at 30xc2x0 C. The polymerization degree is obtained with the following formula:
P=([xcex7]xc3x97103/8.29)(1/0.62)
The degree of saponification of the vinyl alcohol polymer of the present invention is 80 to 99.99 mole %, preferably 84 to 99.9 mole %, more preferably 87 to 99.7 mole %, most preferably 90 to 99.5 mole %. If the degree of saponification is less than 80 mole %, the vinyl alcohol polymer will have markedly low crystallinity, thereby failing to exhibit the high gas-barrier properties, water-vapor-barrier property and water resistance intended by the present invention. In addition, the polymer will have poor thermal stability, and hence cannot be melt molded satisfactorily due to thermal decomposition and gelation.
On the other hand, if the degree of saponification exceeds 99.99 mole %, the vinyl alcohol polymer will not be producible stably and the resulting polymer will not be stably moldable into shapes or films.
The total content of carboxyl group and lactone rings in the vinyl alcohol polymer of the present invention is 0.02 to 0.4 mole %, preferably 0.022 to 0.37 mole %, more preferably 0.024 to 0.33 mole %, most preferably 0.025 to 0.3 mole %. The carboxyl group referred to herein includes its alkali metal salts, such as potassium salt and sodium salt.
If the total content of carboxyl group and lactone rings is less than 0.020 mole %, the polymer will have poor thermal stability, so that its melt moldability decreases due to gelation. In this case, its aqueous solution has low viscosity stability at low temperatures, or its high-concentration aqueous solution has low viscosity stability. In any case, there cannot be obtained the vinyl alcohol polymer intended by the invention. On the other hand, if the total content of carboxyl group and lactone rings exceeds 0.4 mole %, the PVA will have poor thermal stability on melting, so that it cannot be melt molded due to thermal decomposition and gelation. The gas- and water vapor-barrier properties and water resistance of the polymer will also become worse, perhaps due to high affinity with water. Furthermore, the polymer will sometimes become of decreased biodegradability.
It has further been found that the effect of the present invention is markedly enhanced when the total content of carboxyl group and lactone rings satisfies the following condition (1).
xe2x88x921.94xc3x9710xe2x88x925xc3x97P+0.044xe2x89xa6Contentxe2x89xa6xe2x88x921.39xc3x9710xe2x88x924xc3x97P+0.42xe2x80x83xe2x80x83(1)
wherein xe2x80x9cContentxe2x80x9d (in mole %) means the total content of carboxyl group and lactone rings and P represents the viscosity average degree of polymerization of the vinyl alcohol polymer.
The vinyl alcohol polymer comprising ethylene units in the specific amount, as well as carboxyl group and lactone rings may be produced by, for example, the following processes.
{circle around (1)} A process which comprises copolymerizing a vinyl ester monomer such as vinyl acetate with a monomer capable of forming carboxyl group and lactone rings to obtain a vinyl ester polymer and then saponifying the obtained polymer in a solution in alcohol or dimethyl sulfoxide;
{circle around (2)} A process which comprises polymerizing a vinyl ester monomer in the presence of a thiol containing carboxyl group, such as mercaptoacetic acid or 3-mercaptopropionic acid and then saponifying the obtained polymer;
{circle around (3)} A process which comprises, on polymerization of a vinyl ester monomer such as vinyl acetate, effecting chain transfer reaction to the alkyl group of the vinyl ester monomer or vinyl ester polymer, to obtain a highly branched vinyl ester polymer and then saponifying the obtained polymer;
{circle around (4)} A process which comprises reacting a copolymer of a monomer having epoxy group and a vinyl ester monomer, with a thiol having carboxyl group and then saponifying the resulting product; and
{circle around (5)} A process which comprises effecting acetalization of PVA with an aldehyde having carboxyl group.
Examples of vinyl ester monomers usable for producing the vinyl alcohol polymer of the present invention are vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate and vinyl versatate. Among these, vinyl acetate is preferably used for producing PVA.
Examples of monomers capable of forming carboxyl group and lactone rings and usable for producing the vinyl alcohol polymer of the present invention are monomers having carboxyl group, derivable from fumaric acid, maleic acid, itaconic acid, maleic anhydride, itaconic anhydride and the like; acrylic acid and salts thereof; acrylic acid esters, e.g. methyl acrylate, ethyl acrylate, n-propyl acrylate and i-propyl acrylate; methacrylic acid and salts thereof; methacrylic acid esters, e.g. methyl methacrylate, ethyl methacrylate, n-propyl methacrylate and i-propyl methacrylate; acrylamide derivatives, e.g. acrylamide, N-methylacrylamide and N-ethylacrylamide; and methacrylamide derivatives, e.g. methacrylamide, N-methylmethacrylamide and N-ethylmethacrylamide.
The total content of carboxyl group and lactone rings in a vinyl alcohol polymer can be determined from the peaks obtained by proton NMR, as follows. The vinyl alcohol polymer is completely saponified to a degree of saponification of at least 99.95 mole %. The obtained polymer is then sufficiently washed with methanol and vacuum dried at 90xc2x0 C. for 2 days, to yield a sample for analysis.
For the PVA prepared by the above process {circle around (1)}: the vinyl alcohol polymer sample for analysis thus prepared is dissolved in DMSO-D6, and analyzed by 500 MHz proton NMR (with GX-500, JEOL) at 60xc2x0 C. The content of the monomer of acrylic acid, acrylic acid esters, acrylamide or acrylamide derivatives is determined with use of the peak (2.0 ppm) originating from the main chain methyne, and that of the monomer of methacrylic acid, methacrylic acid esters, methacrylamide or methacrylamide derivatives with use of the peaks (0.6 to 1.1 ppm) originating from the methyl groups directly bound to the main chain, by the usual method. For monomers having carboxyl group and derivable from fumaric acid, maleic acid, itaconic acid, maleic anhydride or itaconic anhydride, after the prepared PVA sample for analysis has been dissolved in DMSO-D6, a few drops of trifluoroacetic acid are added to the solution and the resulting solution is analyzed by 500 MHz proton NMR (with GX-500, JEOL) at 60xc2x0 C. The content of lactone rings is obtained by the usual way with use of the peak at 4.6 to 5.2 ppm assigned to the methyne of lactone ring.
For the PVA prepared by the above process {circle around (2)} or {circle around (4)}, the content is obtained with use of the peak (2.8 ppm) originating from methylene bound to sulfur atom.
For the PVA prepared by the above process {circle around (3)}, the prepared PVA sample for analysis is dissolved in methanol-D4/D2O=2/8 and analyzed by 500 MHz proton NMR (with GX-500, JEOL) at 80xc2x0 C. The peaks of 2.2 ppm and 2.3 ppm (integrated values A and B) are assigned to the methylenes of terminal carboxyl group and its alkali metal salt, respective-ly (the following chemical formulas (I) and (II)), the peak of 2.6 ppm (integrated value C) to the methylene of terminal lactone ring (the chemical formula (III)), and the peak of 3.5 to 4.15 ppm (integrated value D) to the methyne of vinyl alcohol units. The total content of carboxyl group and lactone ring is obtained with the following formula:
Total content=50xc3x97(A+B+C)xc3x97(100xe2x88x92xcex94)/(100xc3x97D)
where xcex94 represents the modification ratio (mole %)
Chemical formula (I): (Na) HOOCCH2CH2CH2xcx9c
Chemical formula (II): (Na)OOCCH2CH2CH (OH)xcx9c
Chemical formula (III): 
For the PVA prepared by the above process {circle around (5)}: the prepared PVA sample for analysis is dissolved in DMSO-D6, and analyzed by 500 MHz proton NMR (with GX-500, JEOL) at 60xc2x0 C. The content is obtained in the usual manner with use of the peak of 4.8 to 5.2 ppm originating from the methyne of acetal part (the following chemical formula (IV).
Chemical formula (IV): 
xe2x80x83wherein X represents an alkyl group having 0 to 10 carbon groups.
Within limits not to impair the effect of the present invention, the vinyl alcohol polymer of the present invention may contain monomer units other than vinyl alcohol units, ethylene units, vinyl ester units and the above described monomer units capable of forming carboxyl group and lactone rings. Examples of such other monomers are xcex1-olefins, e.g. propylene, 1-butene, isobutene and 1-hexene; vinyl ethers, e.g. methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether and n-butyl vinyl ether; hydroxyl group-containing vinyl ethers, e.g. ethylene glycol vinyl ether, 1,3-propanediol vinyl ether and 1,4-butanediol vinyl ether; allyl ethers, e.g. allyl acetate, propyl allyl ether, butyl allyl ether and hexyl allyl ether; monomers containing an oxyalkylene group; vinyl silanes, e.g. vinyltrimethoxysilane; hydroxyl group-containing xcex1-olefins, e.g. isopropenyl acetate, 3-buten-1-ol, 4-penten-1-ol, 5-hexen-1-ol, 7-octen-1-ol, 9-decen-1-ol and 3-methyl-3-buten-1-ol; sulfonic acid group-containing monomers, e.g. ethylenesulfonic acid, allylsulfonic acid, methallylsulfonic acid and 2-acrylamide-2-methylpropanesulfonic acid; and cationic group-containing monomers, e.g. vinyloxyethyltrimethylammonium chloride, vinyloxybutyltrimethylammonium chloride, vinyloxyethyldimethylamine, vinyloxymethyldiethylamine, N-acrylamidemethyltrimethylammonium chloride, 3-(N-methacrylamide)propyltrimethylammonium chloride, N-acrylamideethyltrimethylammonium chloride, N-acrylamidedimethylamine, allyltrimethylammonium chloride, methallyltrimethylammonium chloride, dimethylallylamine and allylethylamine. The content of these monomers may differ depending on the purpose or use, but is generally in an amount not more than 20 mole %, preferably not more than 10 mole %.
The vinyl alcohol polymer of the present invention includes terminal-modified PVA obtained by copolymerizing a vinyl ester monomer such as vinyl acetate and ethylene in the presence of a thiol such as 2-mercaptoethanol or n-dodecylmercaptan, except the above-described mercaptan having carboxyl group, and then saponifying the obtained copolymer.
The vinyl ester monomer used and ethylene may be copolymerized by any known process such as bulk polymerization, solution polymerization, emulsion polymerization or suspension polymerization. Among these processes, bulk polymerization with no solvent or in a solvent such as alcohols, or solution polymerization is generally employed. Examples of alcohols usable as solvent on solution polymerization are lower alcohols, e.g. methyl alcohol, ethyl alcohol and propyl alcohol. Examples of initiators usable for the copolymerization are known initiators such as azo-based and peroxide ones, e.g. 2,2xe2x80x2-azobisisobutyronitrile, 2,2xe2x80x2-azobis(2,4-dimethylvaleronitrile), benzoyl peroxide and n-propyl peroxydicarbonate. The polymerization temperature is, with no specific restrictions though, suitably selected within a range of 0 to 150xc2x0 C. However, on setting the polymerization conditions, it is necessary, as obvious from the Examples to be described later, to select various conditions such that the vinyl alcohol polymer intended by the present invention can be smoothly obtained.
The vinyl alcohol polymer of the present invention desirably has a melting point of 160 to 230xc2x0 C., preferably 170 to 227xc2x0 C., more preferably 175 to 224xc2x0 C., most preferably 180 to 220xc2x0 C. With a melting point lower than 160xc2x0 C., the vinyl alcohol polymer tends to have decreased crystallinity, so that satisfactory shaped articles having sufficient water resistance, gas- and water vapor-barrier properties and mechanical strength cannot be obtained therefrom. On the other hand, a high melting point of more than 230xc2x0 C. may render it difficult to produce shaped articles stably from the polymer.
The melting point of a vinyl alcohol polymer means the peak-top temperature of an endothermic peak showing the melting point of the vinyl alcohol polymer when the polymer has been, with DSC, in nitrogen, heated at a temperature elevation rate of 10xc2x0 C./min to 250xc2x0 C., then cooled to room temperature and heated again at a temperature elevation rate of 10xc2x0 C./min to 250xc2x0 C.
The vinyl alcohol polymer of the present invention exhibits biodegradability and is, when subjected to activated sludge treatment or buried in the ground, decomposed into water and carbon dioxide. Continuous treatment of the PVA with activated sludge can decompose the polymer completly in 2 days. From the standpoint of this biodegradability, the vinyl alcohol polymer should be water-soluble or water-dispersible. To this end, the vinyl alcohol polymer preferably has a 1,2-glycol bond content of 1.2 to 2 mole %, more preferably 1.25 to 1.95 mole %, most preferably 1.3 to 1.9 mole %. If the 1,2-glycol bond content of a vinyl alcohol polymer is less than 1.2 mole %, the vinyl alcohol polymer will sometimes have not only poor biodegradability but poor moldability due to too high a melt viscosity. On the other hand, if the 1,2-glycol bond content of a vinyl alcohol polymer exceeds 2 mole %, the polymer will tend to become of poor gas- and water vapor-barrier properties and water resistance, due perhaps to decreased crystallinity of the polymer.
The 1,2-glycol bond content of a vinyl alcohol polymer can, for example, be controlled by using a third component of copolymerization, such as ethylene carbonate, or by adjusting the polymerization temperature. The 1,2-glycol bond content can be determined from the peak of NMR, as follows. A vinyl alcohol polymer sample is saponified to a degree of saponification of at least 99.9 mole %, washed sufficiently with methanol and then vacuum dried at 90xc2x0 C. for 2 days. The PVA sample is then dissolved in DMSO-D6 and, after addition of a few drops of trifluoroacetic acid, tested by 500 MHz proton NMR (with GX-500, JEOL) at 80xc2x0 C.
The peak of 3.2 to 4.0 ppm. (integrated value Axe2x80x2) is assigned to the peak originating from the methyne of vinyl alcohol unit and that of 3.25 ppm (integrated value Bxe2x80x2) to one of the methynes of 1,2-glycol bond. The 1,2-glycol bond content is obtained with the following formula:
1,2-Glycol bond content (mole %)=Bxe2x80x2xc3x97(100xe2x88x92xcex94)/Axe2x80x2
where xcex94 means the ratio of ethylene modification (mole %)
In the present invention, xe2x80x9ca hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expressionxe2x80x9d means the peak (I) reflecting the tacticity of a triad of hydroxyl group protons when a PVA solution in DMSO-D6 is tested by 500 MHz proton,NMR (with GX-500, JEOL) at 65xc2x0 C.
The peak (I) is the sum of, in terms of triad expression of hydroxyl groups of the PVA, an isotacticity chain (4.54 ppm), a heterotacticity chain (4.36 ppm) and a syndiotacticity chain (4.13 ppm), while the peak (II) originating from hydroxyl groups in all vinyl alcohol units apears in a region of chemical shift 4.05 ppm to 4.70 ppm. The molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression is, in the present invention, obtained by 100xc3x97(I)/(II).
In the present invention, it has been found that by controlling at an appropriate level the amount, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain of a vinyl alcohol polymer, the following properties of the polymer can be controlled. The properties include those related to water, e.g. water solubility, moisture absorbing capability, water resistance and barrier properties and those related to melt moldability, e.g. melting point, melt viscosity and melt flowability. This is attributable to the fact that the hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain has good crystallinity and hence enables the features of the vinyl alcohol polymer to develop sufficiently.
The content of a hydroxyl group of vinyl alcohol unit in 3 successive vinyl alcohol unit chain in terms of triad expression in the vinyl alcohol polymer of the present invention is preferably 65 to 98 mole %, more preferably 70 to 97.5 mole %, still more preferably 72 to 97 mole %, still more preferably 74 to 96 mole %, and most preferably 75 to 95 mole %.
If the content of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression of a vinyl alcohol polymer is less than 65%, the polymer will tend to have markedly low crystallinity. Then, the polymer will not exhibit the high gas- and water-vapor-barrier properties and water resistance. Also, the polymer will have poor thermal stability, thereby becoming unable to undergo satisfactory melt molding. Furthermore, the mechanical properties, which are key features of vinyl alcohol polymers in general, of the obtained films, shaped articles are impaired. On the other hand, if the content of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression of a vinyl alcohol polymer exceeds 98 mole %, the polymer will tend to have markedly high crystallinity. As a result, preparation of its aqueous solution requires much labor. Furthermore, the polymer has high melting point, which requires high melt molding temperature. The polymer then has poor thermal stability on melt molding and readily decomposes or gels, causing severe coloration.
It has also been found that the effect of the present invention is markedly enhanced when the vinyl alcohol polymer of the present invention has such a molar fraction, based on vinyl alcohol units, of a hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression, that satisfies the following formula (2):
xe2x88x921.5xc3x97Et+100xe2x89xa7molar fractionxe2x89xa7xe2x88x92Et+85xe2x80x83xe2x80x83(2)
wherein xe2x80x9cmolar fractionxe2x80x9d (in mole %) means the molar fraction, based on vinyl alcohol units, of the hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain in terms of triad expression, and Et represents the content (in mole %) of ethylene in the vinyl alcohol polymer.
The ethylene-modified vinyl alcohol polymer composition of the present invention has the key feature of containing an alkali metal. The content of the alkali metal (B) in terms of sodium (B) based on 100 parts by weight of the vinyl alcohol polymer (A) is 0.0003 to 1 part by weight, preferably 0.0003 to 0.8 part by weight, more preferably 0.0005 to 0.6 part by weight, most preferably 0.0005 to 0.5 part by weight. Examples of the alkali metal are potassium and sodium, which are mainly present as a salt of a lower aliphatic acid such as acetic acid or propionic acid, a salt of the carboxyl group contained in the PVA of the present invention and containing a specific amount of carboxyl group, or a salt of a sulfonic acid which may be contained in a copolymerization monomer. The alkali metal may also be present in an additive to the composition.
If the content of an alkali metal is less than 0.0003 part by weight, the water solubility when the vinyl alcohol polymer is used in the form of an aqueous solution will decrease and the polymer does not develop satisfactory features. Where the vinyl alcohol polymer is used under melting conditions, the polymer gels severely on melting, thereby exhibiting poor melt moldability. As a result, not only the productivity decreases, but the resultant shaped articles do not have sufficient water solubility. On the other hand, if the content of an alkali metal exceeds 1 part by weight, the polymer will show poor thermal stability on melting and severely decompose, gel or color, thereby becoming unable to be molded into shapes. Furthermore, the gas- and water vapor-barrier properties become worse perhaps due to decrease of the crystallinity.
In the present invention, there are no specific restrictions with respect to the process of incorporating the specific amount of an alkali metal (B). There may thus be employed a process which comprises adding a compound containing an alkali metal to the once obtained vinyl alcohol polymer or, preferably, a process which comprises, on saponification of a vinyl ester polymer in a solvent, using as a saponification catalyst an alkaline substance containing an alkali metal, thereby incorporating the alkali metal in the resulting vinyl alcohol polymer and then washing the saponified vinyl alcohol polymer with a washing liquid to control the content of the alkali metal in the resulting vinyl alcohol polymer. This latter process is preferred.
The content of an alkali metal is determined by atomic absorption photometry.
The vinyl alcohol polymer of the present invention, containing a specific amount of ethylene units, may be produced by any known process such as one which comprises copolymerizing ethylene and a vinyl ester monomer to obtain a vinyl ester polymer and then saponifying the polymer in an alcohol or dimethyl sulfoxide.
Examples of alkaline substances usable as a saponification catalyst are potassium hydroxide and sodium hydroxide. The molar ratio of the alkaline substance used as a saponification catalyst is, based on vinyl acetate units, preferably 0.004 to 0.5, more preferably 0.005 to 0.05. The saponification catalyst may be added all at once at an early stage of saponification reaction or further added during the reaction.
The saponification is conducted in a solvent such as methanol, methyl acetate, dimethyl sulfoxide or dimethylformamide. Of these solvents, preferred is methanol, more preferably methanol having a water content of 0.001 to 1% by weight, still more preferably methanol having a water content of 0.003 to 0.9% by weight, most preferably methanol having a water content of 0.005 to 0.8% by weight. The saponification is preferably carried out at a temperature of 5 to 80xc2x0 C., more preferably 20 to 70xc2x0 C. and preferably for 5 minutes to 10 hours, more preferably for 10 minutes to 5 hours. The saponification may be conducted by a known process such as batch process or continuous process.
As examples of the washing liquid to be used after the saponification, there may be mentioned methanol, acetone, methyl acetate, ethyl acetate, hexane and water. Among these, use of methanol, methyl acetate or water, either singly or in combination, is preferred.
The amount of the washing liquid is adjusted such that the content of an alkali metal (B) falls within the satisfactory range and is generally, based on 100 parts by weight of PVA, 30 to 10,000 parts by weight, more preferably 50 to 3,000 parts by weight. The washing temperature is preferably 5 to 80xc2x0 C., more preferably 20 to 70xc2x0 C. The washing time is preferably 20 minutes to 10 hours, more preferably 1 hour to 6 hours. The washing may be conducted batch-wise, by counter-current washing or by other known processes.
The vinyl alcohol polymer and its composition of the present invention can be obtained, as is apparent from the description of the Examples to be given later, by properly selecting the polymerization conditions (polymerization temperature, amounts of vinyl acetate, solvent and ethylene, successive addition conditions of ethylene and initiator, ethylene pressure on polymerization, polymerization time, conversion, and addition conditions of delay such as maleic anhydride) and the saponification conditions (concentration of polyvinyl acetate, amount of alkali, temperature, time, neutralization conditions and washing conditions). If appropriate conditions were not selected, the intended vinyl alcohol polymer or its composition could not be obtained.
The vinyl alcohol polymer of the present invention, having a specific ethylene content, polymerization degree, degree of saponification, and specific amounts of carboxyl group and lactone rings, 1,2-glycol bond and 3 successive vinyl alcohol unit chain, and its composition comprising a specific amount of an alkali metal have markedly better thermal stability compared to conventional PVA and are useful as resins for melt molding.
The amount of gel generated when a resin is heat treated at a temperature for a certain time is taken as an index of the thermal stability of the resin. With the vinyl alcohol polymer and its composition of the present invention, the gel generation is small compared to conventional PVA. This fact means that the vinyl alcohol polymer and its composition of the present invention are markedly excellent in melt moldability in commercial operations, which should be run stably over a long period of time.
The mechanism involved in the development of high thermal stability by the ethylene-modified PVA and its composition of the present invention is not clear. It is, however, estimated that the key role is played by the ethylene-modified PVA having a specific polymerization degree and degree of saponification, and specific amounts of carboxyl group, lactone rings, 1,2-glycol bond, hydroxyl group chain and an alkali metal, in particular having a specific ethylene modification ratio and specific amounts of carboxyl group and lactone rings.
The vinyl alcohol polymer or the composition of the present invention may, within limits not to impair the purpose and effect of the present invention, incorporate as necessary the usual additives, e.g. filler, processability stabilizer such as copper compound, weatherability improving agent, color, UV absorber, light stabilizer, antioxidant, antistatic, flame retardant, plasticizer, other thermoplastic resins, lubricant, perfume, foaming agent, deodorant, extender, release agent, reinforcing agent, mildew proof agent, antiseptic and crystallization retarding agent. In particular, addition of, as a thermal stabilizer, organic stabilizers, e.g. hindered phenol, copper halides such as copper iodide, and alkali metal halides such as potassium iodide is preferred, since the addition improves stability during dwell time when the vinyl alcohol polymer is being melt molded.
Among the above additives, plasticizers have the effects of decreasing the melting point, thereby further improving the melt moldability and providing the obtained molded products with flexibility and toughness. Any plasticizer may be used for this purpose, insofar as it can decrease the glass transition point or melt viscosity of the PVA. Examples, but with no limitations thereto, of usable plasticizers include, but not limited to, glycerine; diols, e.g. 1,3-butanediol and 2,3-butanediol; glycols, e.g. ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, polypropylene glycol, trimethylene glycol, tetramethylene glycol, penta methylene glycol, hexamethylene glycol and propylene glycol; glycerine derivatives, e.g. polyhydric alcohols such as sorbitol, glycerine and diglycerine, to which ethylene oxide or propylene oxide has added; sorbitol; pentaerythritol; saccharides; polyethers; phenol derivatives, e.g. bisphenol A and bisphenol S; amides, e.g. N-methylpyrrolidone; trimethylolpropane; diglycerine; 3-methyl-1,3,5-pentanetriol and a small amount (not more than 20%) of water. These plasticizers may be added in an amount based on 100 parts by weight of the vinyl alcohol polymer of preferably not more than 30 parts by weight, more preferably 20 parts by weight and most preferably 10 parts by weight.
Examples of other thermoplastic polymers that may be used in combination of the vinyl alcohol polymer of the present invention are general-purpose resins such as polyethylene, polystyrene and ABS.
Among the above additives, fillers have the effects of increasing the hardness and rigidity to provide a solid hand, developing blocking-preventing property and controlling the rate of water-degradation or biodegradation. Examples of usable fillers are known inorganic fillers, e.g. kaolin, clay, talc, acid clay, silica, alumina, diatomaceous earth, bentonite, montmorillonite, knot clay, agalmatolite, alunite, porcelain clay, faldspar, graphite, pearlite, calcium carbonate, magnesium hydroxide, carbon black, titanium oxide, mica, zirconia, boron nitride, aluminum nitride, shirasu, glass and glass fiber; and organic fillers, e.g. urea-formaldehyde resin and melamine-formaldehyde resin. The average particle size of inorganic fillers usable in the present invention is, with no specific limitation though, preferably 0.1 to 100 xcexcm.
Although there is no particular limitation to the amount of fillers added, it is preferably not more than 400 parts by weight based on 100 parts by weight of the vinyl alcohol polymer, more preferably not more than 200 parts by weight on the same basis.
Where the above additives are incorporated into the vinyl alcohol polymer of the present invention, there are no specific restrictions with respect to the process of addition. These stabilizer, plasticizer, filler and other additives may be simply added to the vinyl alcohol polymer, or melt kneaded therewith to form pellets. Thus, these additives and the vinyl alcohol polymer may separately be fed to a melt kneader at constant rates, to undergo kneading and pelletization.
The vinyl alcohol polymer of the present invention may be melt molded by, for example, extrusion molding, injection molding, film formation by extrusion through a T-die, tubular film process, compression molding, transfer molding, molding into reinforced plastics, hollow molding, pressing, blow molding, calendering, foaming, vacuum forming and pressure-vacuum molding. Another thermoplastic resin may as desired be laminated with the vinyl alcohol polymer of the present invention.
Naturally, the vinyl alcohol polymer of the present invention can, in the form of a solution in a solvent such as water or dimethyl sulfoxide, be processed into shaped articles.
These processes can give shaped articles having any optional shape, such as film, sheet, tube or bottle.
The vinyl alcohol polymer of the present invention, having a specific ethylene content, polymerization degree, degree of saponification, and specific amounts of carboxyl group, lactone rings, 1,2-glycol bond and 3 successive vinyl alcohol unit chain, and its composition comprising a specific amount of an alkali metal further have the feature of exhibiting excellent oxygen-barrier property and are hence useful as resin for oxygen-barrier films.
That is, the vinyl alcohol polymer and its composition of the present invention have a small oxygen transmission rate as determined by the specific method shown below, which is an index of oxygen-barrier property, compared to conventional PVA. The mechanism involved in the development of high oxygen-barrier property by the ethylene-modified PVA and its composition of the present invention is not clear. It is, however, estimated that the key role is played by the ethylene-modified PVA having a specific polymerization degree and degree of saponification, and a specific amount of carboxyl group and lactone rings, 1,2-glycol bond, hydroxyl group of vinyl alcohol unit located at the center of 3 successive vinyl alcohol unit chain and an alkali metal, in particular having a specific ethylene modification ratio, a specific amount of carboxyl group and lactone rings, and a specific alkali.
The oxygen transmission rate is determined as follows. A film obtained from a vinyl alcohol polymer is heat treated in air (preferably at 100 to 240xc2x0 C., more preferably at 120 to 240xc2x0 C.; for preferably 10 to 300 seconds, more preferably 30 to 180 seconds) and, after conditioning at 20xc2x0 C., 80% RH, tested for the oxygen transmission rate. The obtained value is then converted into the one corresponding to a film thickness of 20 xcexcm.
For a laminate sample, the sample is, as it is, measured for the oxygen transmission rate, which is then converted into the rate corresponding to a vinyl alcohol polymer layer thickness of 20 xcexcm.
Laminates exhibit oxygen-barrier property because they have been subjected to some kind of heat treatment. Further heat treatment is therefore not necessary on determination of oxygen transmission rate for laminate samples.
The oxygen-barrier property of a base film for a laminate is very low compared to that of a film comprising the vinyl alcohol polymer, and hence the oxygen-barrier property of the laminate is substantially governed by that of the vinyl alcohol polymer film constituting the laminate. For this reason, even for a laminate it is possible to obtain an oxygen transmission rate corresponding to the constituting vinyl alcohol polymer film having a thickness of 20 xcexcm. Hereinbelow, the oxygen transmission rate means, unless otherwise indicated, that converted into a value corresponding to a PVA film thickness of 20 xcexcm.
The oxygen-barrier property in terms of oxygen transmission rate as determined by the above method is, for practical purposes, preferably not more than 15 cc/m2.day.atm, more preferably not more than 10 cc/m2.day.atm, most preferably 5 cc/m2.day.atm.
The vinyl alcohol polymer may be applied onto a base film to form laminates. Examples of the base film are polyolefin films, polyester films and polyamide films.
These base films preferably have a thickness (when the laminates are stretched, the thickness of the finished base films) of 5 to 100 xcexcm.
On producing a laminate by applying the vinyl alcohol polymer of the present invention on a base film, use of a crosslinking agent in combination is desirable, although the vinyl alcohol polymer may be used alone. Examples of the crosslinking agent are epoxy compounds, isocyanate compounds, aldehyde compounds, silica compounds, aluminum compounds, zirconium compounds and boron compounds, of which silica compounds, such as colloidal silica and alkyl silicates are preferred. These crosslinking agents are added in an amount based on 100 parts by weight of the vinyl alcohol polymer of, generally 5 to 60 parts by weight, preferably 10 to 40 parts by weight, more preferably 15 to 30 parts by weight. A high addition exceeding 60 parts by weight sometimes impairs the oxygen-barrier property.
On producing a laminate by applying the vinyl alcohol polymer on a base film, the polymer is generally applied in the form of an aqueous solution. There is no particular limitation to the concentration of the aqueous solution, but it is desirably 5 to 50% by weight. A low concentration of less than 5% by weight causes the drying load to increase, while a high concentration exceeding 50% by weight increases the viscosity of the aqueous solution, thereby decreasing the applicability.
The aqueous solution of the vinyl alcohol polymer may, on application, contain a surface active agent, a levelling agent and the like. The aqueous solution may also contain up to about 30% by weight of a lower aliphatic alcohol such as methanol, ethanol or isopropyl alcohol, which leads to better applicability.
The aqueous solution of the vinyl alcohol polymer may further contain a mildew-proof agent, antiseptic and the like. The aqueous solution of the vinyl alcohol polymer is applied at a temperature of preferably 20 to 80xc2x0 C. The application is conducted suitably by gravure roll coating, reverse gravure coating, reverse roll coating or Meyer bar coating.
With respect to the timing of when to apply an aqueous solution of the vinyl alcohol polymer, the application may be conducted either after the base film has been stretched or heat treated or before such treatment. In view of operability, it is desirable to carry out the steps of subjecting a base film to a first-stage stretching, applying the aqueous solution and then subjecting the film with the solution to a second-stage stretching, during or after which conducting heat treatment of the laminate. The thickness of the vinyl alcohol layer (final thickness when stretching is conducted) is preferably 0.1 to 20 xcexcm.
An adhesive component layer may be formed to improve the adhesiveness, between a film layer comprising the vinyl alcohol polymer and a base film layer. The adhesive component can be applied on the surface of the base film before application of an aqueous solution of the vinyl alcohol polymer, or may be mixed into the aqueous solution and used.
Gas-barrier laminated films comprising a film layer of the vinyl alcohol polymer generally further have a layer of a heatsealable resin on the polymer layer. The heatsealable resin layer is generally formed by extrusion lamination or dry lamination. Examples of the heatsealable resin are polyethylene resins, e.g. HDPE, LDPE and LLDPE, polypropylene resin, ethylene-vinyl acetate copolymer, ethylene-xcex1-olefin random copolymers and ionomer resins.
There are no specific restrictions with respect to whether stretching is conducted or not, the heat treatment temperature and like conditions. Usually, after application of the vinyl alcohol polymer on an oriented film of a polyolefin, polyester or polyamide at a temperature suitable for the pertinent resin, the laminate is heat treated in air or the like. The suitable heat treatment temperature is 140 to 170xc2x0 C. for polyolefin base films and 140 to 240xc2x0 C. for polyester and polyamide base films. The vinyl alcohol polymer film layer is usually heated treated at the same time with the heat treatment of the base film used.
The vinyl alcohol polymer of the present invention, having a specific ethylene content, polymerization degree and degree of saponification, and a specific amount of carboxyl group and lactone rings, 1,2-glycol bond and 3 successive vinyl alcohol unit chain, and its composition can be used for examples as the following items. That is, they are usable as sizing agent for fibers, fiber treating agent, fiber processing agent, sizing agent for textile products, paper processing agents, e.g. clear-coating agent for paper, pigment-coating agent for paper, sizing agent to be added to slurry for producing paper, and binder for overcoating of heat-sensitive paper, heat-sensitive adhesive, defogging agent, paints, dispersing agent for organic and inorganic pigments, dispersion-stabilizing agent for emulsion polymerization, dispersion-stabilizing agent for polymerization of vinyl chloride, adhesive for paper, wood and plastics, binder for nonwovens, binder for fibers, binder for ceramics, binder for various construction materials such as gypsum board and fiber board, additive for cement and mortar, hot-melt adhesive, image-forming material, photosensitive resin, raw material for polyvinyl acetal for formal resins and butylal resins, substrate for gel, raw material for shaped articles such as films, sheets and tubes and soil conditioner. The vinyl alcohol polymer of the present invention may, utilizing its features, be either used alone or in combination with other polymers, e.g. unmodified or modified PVAs, starch (and its modified products), cellulose derivatives, gums, gelatin and casein; and plasticizers.